1. Field of the Invention
The present invention relates to the production of nitrogen by cryogenic separation of air in a single distillation column process.
2. Discussion of the Background
The production of nitrogen by cryogenic separation of air in a single column process is widely used at present. The conventional process affords nitrogen at pressures of about 5-8 bar. With this process, liquid nitrogen is obtainable, however, recovery is limited by equilibration at the bottom of the column. Generally, this process allows for the recovery of about 50-60% of the nitrogen in the air feed. The required refrigeration for the process is obtained by expanding the waste stream from about 2-5 bar to atmospheric pressure.
It would be extremely desirable to use such a process to produce nitrogen, at higher recoveries and lower pressures of about 1.5-4 bar, however, it is not feasible at present to use the conventional single-column process for the production of nitrogen at such lower pressures for a variety of reasons.
First, a low nitrogen pressure results in a low waste pressure. This is especially problematic for plants of small size, whereby waste expansion is no longer sufficient to provide the required refrigeration. Moreover, liquid production would be difficult.
Second, a low nitrogen pressure also means a low air pressure at the inlet of the cold box. At low pressure the removal of water vapor and carbon dioxide becomes expensive and is not economically feasible.
Third, although a single distillation column process for the production of nitrogen would, in theory, produce an oxygen-enriched stream, as a waste stream, the conventional process cannot be used to produce an oxygen-enriched stream under pressure since it would result in significant back pressure at the outlet of the expander.
Thus, a need clearly continues to exist for an economical process for the production of nitrogen of high purity and high recovery at lower nitrogen pressures and with the capability of producing a small amount of liquid product.